The present invention relates to wireless communication technology, in particular, relates to power saving technology of a base station in a wireless communication system.
With prevalence of wireless data communication, installment of a wireless base station is progressing to widen a service area. A cover range, where one wireless base station is possible to perform wireless communication with a wireless terminal, is called “cell”. Usually, a wireless base station is installed at a place with a relatively good view, so as to have as wide cell radius as possible. The cover range of a wireless base station is several-hundred meters in a densely-populated area with many buildings, and several-ten kilometers in a place with many fields, however, even in a cell, there may occur a place where a radio wave does not reach or is difficult to reach from a wireless base station, such as inside or underground of a building, or a place behind a building.
On the other hand, a system of a wireless base station covering a very small range, called a femtocell, has been noticed recently. The femtocell has a cell radius of about several meters, and such utilization way is assumed that covers one home or one small office by one wireless base station.
The femtocell is connected to a fixed line (such as a telephone line, an optical fiber or a cable for a CATV) which is connectable to a telephone line in a home, so as to perform communication with a wireless terminal. Therefore, it is expected to cover very finely to the indoor or underground, which has been difficult to cover by a conventional wireless base station. In addition, it is possible to cover also a mountain area where installment of a wireless base station was difficult. In order to connect to a femtocell base station, there is required no change at a wireless terminal side, and connection is possible by a terminal just like a conventional one. In addition to this, there is a merit, for a carrier, that a back bone to provide a service can be strengthened.
As described above, because the femtocell is a system of a wireless base station having a cell radius of about several meters, and the femtocell is utilized by being placed in a home, the number of users connectable is several users at most. However, it is required to be capable of communicating at all times, similarly to a usual wireless base station.
A transmitter of the usual wireless base station keeps transmitting a pilot signal at all times to a wireless terminal existing in a service area. In addition, also a receiver of the wireless base station is in an activation status at all times, and takes a receivable status of a call request signal from a wireless terminal any time.
On the other hand, each wireless terminal receives a signal from a wireless base station by starting a receiver in a constant cycle, to confirm presence or absence of a connection request signal, called paging, addressed to a local terminal, in a received signal. A wireless terminal makes a receiver OFF, in the case where there is no paging addressed to the local terminal. A transmitter of a wireless terminal is always OFF when paging is absent.
In this way, a transmitter of the usual wireless base station keeps transmitting a pilot signal at all times, irrespective of presence or absence of a wireless terminal in a service area, or presence or absence of a call request. In addition, a receiver of the usual wireless base station is in an activation status at all times, so as to receive a call request from a wireless terminal.
However, in the femtocell base station, which is assumed to be installed in a home, it is considered such situation that, in the day time of week days, users are absent and a wireless terminal is absent in a service area, or in midnight or the like, although a wireless terminal is existing in a service area, it is in a continuously standby status and there is little connection to a base station, therefore, an activation status of a transmitter and receiver at all times, by same operation as in the usual wireless base station, results in only useless power consumption. Therefore, in the femtocell base station, development of technology for low power consumption has been required.
As a method for suppressing the power consumption of a wireless base station, there is technology, for example, described in JP-A-2002-158609. In this invention, such a method for shifting to a power saving mode has been proposed that a wireless base station transmits a shift signal to a power saving mode, to a wireless terminal in a service area, and by negotiation between the wireless base station and the wireless terminal, start timing of a receiver of the wireless terminal is made the same, so as to transmit a signal from the wireless base station in matching to that timing. In addition, such a method has also been proposed that a receiver of the base station also performs ON/OFF in matching to a transmitter.